We’ve all heard it said…”reduce the throttle by no more than 1-inch every minutes to ensure you don’t shock-cool your engine”. Does this advice apply to a PA46 engine? Can a PA46 engine (Lycoming 540 or Continental 520/550) really be shock-cooled? How should the engine temperature be managed?
Metals expand and contract with temperature, and the various metals in an air-cooled aviation engine expand and contract at a different rates. Shock-cooling supposedly occurs when the engine changes temperature quickly and the different metals in the engine cool (and therefore change shape) at different rates. When the change occurs dramatically supposed scoring, rubbing, and marking of the metal can occur, which can cause catastrophic results.
So, let’s back to the original question…can a PA46 engine suffer shock-cooling and should a pilot operate the engine so as to avoid shock cooling? Simply put, I’ve never seen nor heard of any piston PA46 engine suffer shock-cooling. In 5000+ hours flying the piston PA46 and 16 years of flying/managing/training in the Malibu/Mirage/Matrix, it simply has not happened to me nor anyone I know. Does it mean that it cannot happen or has never happened? No. But, it is certainly not a prolific threat to our fleet.
Should the owner/pilot operate the engine with a cautious eye cast toward the potential of shock cooling? Well, sort of…but, let’s flesh this out. My suggestion is that a pilot should operate the engine with conservatism in movement of temperature, but only because this is a good operating practice with any machine, and any flying machine is (by definition) not “overbuilt”. And, there are many ways to change the temperature of the engine…not just by reducing power. Here’s a partial list of ways to cool your PA46 engine:
- Reduce power: Obvious…yes. When the engine produces less power, less heat is generated. Reducing power in a piston engine will almost always result in less temperature.
- Lower the nose: By descending (and leaving power in a cruise setting) the airspeed will increase and cool the engine.
- Enriched the mixture: Fuel has a cooling effect on the engine, so the richer the mixture the cooler the engine.
- Lower the landing gear: Yes…you read that right…engine cooling will occur when you lower the landing gear because more air will flow over the cylinders. Notice the landing gear doors on the PA46 have air louvers. Air flows into the engine nacelle on the front, passes down through the cylinders (along with the oil cooler, intercoolers, and other components) and then out the louvers of the closed gear doors. When the landing gear is lowered the “back door is opened” and a LOT more airflows over the cylinders.
My suggestion is that a pilot only perform ONE of these actions at a time when beginning a descent. This suggestion was presented to me by Chad Menne (Owner, Malibu Aerospace) some time ago and I’ve operated engines this way ever since. If you are at a higher altitude and simultaneously reduced the power, lowered the landing gear, started a big descent, and enrichened the mixture in one flail swoop, I think there’s a chance that your engine would suffer some negative effects that could be called “shock cooling”. So, when you do start a descent, pick one “cooling action” to accomplish at a time. I’m sure you’ll not hurt your engine.
Simply put, shock cooling is not a huge factor in the PA46 community, and a PA46 pilot does not need to be overly cautious. The “one inch per minute” rule may apply in some other airframes, but in the PA46 world it is not applicable.